Detection and mapping of ghost-rock features in the Tournaisis area through geophysical methods - an overview

Abstract

In the context of cover-collapse sinkholes, as in the Tournaisis area, the detection of palaeokarst features allows to delineate, at a local scale, areas that might be subject to future collapse. These areas should therefore be avoided or dealt with in the perspective of earthworks for civil engineering. Before the last decade, karst mapping in this context was often wrongly considered as a cavity detection problem and tackled in this way. A better understanding of these phenomena allows developing more appropriate detection methods. We precise the geological settings and karst context of the Tournaisis. We present the key characteristics of the palaeokarsts and the main contrasts in petrophysical properties expected between alterite (ghost-rock) and bedrock. Given these contrasts and the wide range of overburden thicknesses found in the Tournaisis, we evidence the main advantages and major limitations of potentially effective geophysical investigation methods. We illustrate our purpose by describing some results of electrical resistivity tomography and seismic surveys applied to palaeokarst detection. These results are assessed using numerous geotechnical data and geological knowledge. Finally, we show how these results could be used for geological and geotechnical problem solving in particular trough the design of adapted detection and mapping strategies. Detecting and mapping palaeokarst features that lie hidden under a soil or overburden is of importance in several cases, such as in karst aquifer vulnerability assessment, quarry works planning, karst risks mitigation. In the context of cover-collapse sinkholes, the detection of these features allows to delineate at a local scale areas that might be subject to future collapse and should be avoided or dealt with. In the perspective of earthworks for civil engineering works, such information may be of paramount importance at the planning stage to position the works or to select suitable and effective technical solutions. In the same way, in limestone quarry works, overburden removal, groundwater management strategy and operation planning might also benefit from a better knowledge of weathering spatial distribution and intensity.